1. Field of the Invention
The present invention relates to an abnormal voltage detector apparatus for use in an assembled battery, and in particular, to an abnormal voltage detector apparatus for detecting voltage abnormality in the assembled battery.
2. Description of the Related Art
A sealed nickel-metal hydride battery (hereinafter referred to as a “nickel-hydrogen battery”) is excellent in basic characteristics, such as energy density, output density and cycle life. Accordingly, in recent years, attention has been given to such a nickel-hydrogen battery as a power source for motors and as a drive source for various kinds of loads in electric vehicles, such as pure electric vehicles (PEVs) and hybrid electric vehicles (HEVs). Development has been thus advancing to make such a nickel-hydrogen battery practical.
When the nickel-hydrogen battery is used as a power source for electric vehicles, a total voltage of approximately 100 V to 350 V is required to obtain a predetermined drive output voltage. The output voltage of a cell, which is the minimum unit constituting the nickel-hydrogen battery, is approximately 1.2 V. Therefore, an assembled battery including a plurality of battery blocks, each battery block including at least one cell, is used to obtain a desired total voltage.
The temperatures of the cells constituting the assembled battery are not uniform. In particular, in such an environment that the assembled battery is used in a vehicle, temperature differences may occur among the cells. Furthermore, the remaining capacity and the charging efficiency (the ratio of the charged electric quantity to the supplied electric quantity) of respective cells are different from each other depending on production process and usage conditions after the production. For these reasons, the cells constituting the assembled battery have variations in the actual remaining capacity (SOC: state of charge), and the range of the capacity usable as the capacity of the assembled battery is narrowed. In other words, the service life of the assembled battery is apparently shortened significantly. In the assembled battery, it is important to detect the voltage of each cell or battery block constituting the assembled battery, and to judge whether or not the voltage is abnormal to carry out charging or discharging control.
The Japanese patent laid-open publication No. 2003-303626-A discloses an abnormal detector apparatus for an assembled battery according to a prior art. The abnormal detector apparatus according to the prior art includes abnormal detector circuits, each of which provided in each battery block that constitutes the assembled battery and detects whether or not a voltage between terminals is abnormal such as overcharged or overdischarged. The abnormal detector apparatus according to the prior art transmits an abnormal voltage detection signal to a charge and discharge controller side when at least one of the abnormal detector circuits detects that the voltage is abnormal.
According to the abnormal detector apparatus according to the prior art, if one abnormal detector circuit fails and cannot detect an abnormal voltage even when, for example, a corresponding battery block has an overvoltage, it is such a possibility that the assembled battery is continuously used without any user's knowledge of the failure of the abnormal voltage detector circuit and that the battery block is eventually overcharged.
The Japanese patent laid-open No. 9-159701-A discloses overvoltage detector apparatus for use in an assembled battery according to another prior art. The overvoltage detector apparatus according to this prior art can detect overvoltage of each cell or battery block that constitutes the assembled battery and can determine whether an overvoltage detection function is normal or abnormal.
The abnormal detector apparatus and overvoltage detector apparatus according to the prior art have one threshold voltage, respectively, which is a boundary value at which the voltage is abnormal or not, for detecting whether or not the battery block is abnormal. Due to this, even if one battery block that constitutes the assembled battery has a voltage closer to the threshold voltage and is about to be overdischarged or overcharged, for example, the abnormal voltage detection signal is not outputted to the outside unit. As a result, a notification that the battery is abnormal is suddenly displayed at the moment when the voltage of the battery block exceeds the threshold voltage or falls below the threshold voltage.
The abnormal detector apparatus and overvoltage detector apparatus according to the prior art cannot detect voltage abnormality of the assembled battery accurately and reliably when a voltage of the battery block fluctuates with elapsed time.